The present invention relates to the preservation of fruit and vegetable particles and pertains particularly to such preservation by dehydration.
Preservation by evaporation of moisture is an ancient art, and has advantages over other methods. Weight is reduced, making transportation and storage less expensive; the food may be kept in temperature climates for long periods without refrigeration; the flavor is concentrated and pleasing for that reason, in raisins, dates, slices of onions, apples, and the like.
The development of dehydrating methods to the present time is set forth in FIG. 1. Curves 30-31 show the time of exposure to the weather for traditional methods whereby the particles are laid out in the sun on trays to dry for four or more days, during which time they also collect dust and are usually darkened by the sun's ultraviolet rays. The moisture content is reduced by evaporation from the initial value of about 80% to the range of 25% to 15%. Despite the concentrations in sweetness that accrued as the result of this drying, objections arose to the products for their darkened color, and the dust and dirt they accumulated. Some consumers also found the caramelized quality of fruit sugars that was generated to a small extent in some products to be objectionable, and this caramelization was found to be due to prolonged overheating in the hot sun.
Enclosed kilns were developed that blew heated dry air at about 65.degree. C. over the fruit on trays, and between particles laid out on a perforated kiln floor. To preserve the natural color, especially of apple slices, apricot and pear halves and Thompson seedless raisins, the particles were treated to heavy doses of sulfur dioxide, and also sometimes were dipped in solutions of sulfites and other chemical preservative. The result was an attractive product for eye-appeal, but many people who ate these products developed an aversion to the taste of the sulfur compounds that were associated with the food.
Strong objections to toxic concentrations of sulfur compounds were written into law that set the maximum permissible SO.sub.2 level at a low concentration that made prolonged storage at room temperature impossible without darkening, especially in apples and apricots. It was found that refrigerated storage prevented dark colors from developing. The added cost of refrigerated storage made dried fruits uncompetitive with refrigerated fresh fruits except when the storage period was longer than the refrigerated storage life of fresh fruits, as in the spring and early summer in the Northern Hemisphere.
Refrigerated shipments of fresh fruits from Australia and other Southern Hemisphere fruit and vegetable-growing areas have recently made fresh fruits available in the United States all year round, but the price of such produce on the fresh fruit markets has often been prohibitive to mass marketing, and only the fancy food stores can sell such imported fruits at a profit. Bananas can be shipped from Central America and sold at a profit at a relatively low price because of the steady demand and steady production rate of this fruit that keeps refrigerated ocean transports continually busy. However, bananas are also grown on the Hawaiian Islands that are not shipped to the Mainland. This is because Hawaiian bananas are of a variety that do not sustain high quality even in refrigerated storage, and they deteriorate in a very few days after picking. The same applies to Hawaiian pineapple, which is most tasty when fully ripe, but which cannot reach the Mainland by boat in the prime condition, and must be shipped green and ripen on the grocery store shelf, a method not conducive of the development of the high pineapple flavor potential.
Flavor is all-important, and the secret of success in fruit and vegetable marketing is to provide a product that has fully matured before harvesting, and preserve that flavorful product at low cost, and transport it in large quantities to sell at low unit profit on the mass markets.
Canned and frozen fruits and vegetables have met that criterion for many years, but recently the increases in labor and energy costs for sterilizing, for refrigeration, and for the manufacture of steel sheets for cans, have sharply increased the retail prices of these goods.
We have found that the best way to provide the consumer with a high-quality fruit and vegetable product at lowest prices is through dehydration and puffing the product by the method of the present invention, which is an improvement over other attempts to provide an acceptable low-moisture fruit or vegetable.
Improvements in process that resulted in a light-colored low-moisture product of about 31/2% moisture content and low-sulfur content were proposed by C. C. Moore in U.S. Pat. Nos. 1,543,948 issued June 30, 1925, and 2,023,536 issued Dec. 10, 1935. Moore imparted heat to the fruit in a vacuum environment under the conditions of FIG. 1 curve 32, and in about four hours reduced the sulfur to a few hundred ppm, and the moisture to the range where the fruit solids were hard and brittle when cooled. However, this process did not produce an attractive, easily reconstituted comestible, for the product such as apple slices, apricot halves, grapes, and the like dried by that process were exceedingly tough when cold. Moore also dehydrated fresh fruit in accordance with the condition of curve 34.
Moore's method was improved upon by a method that produced a tender puffed fruit of about 2% to 3% moisture content by briefly heating the product rapidly to create pores within the particles, then by cooling and hardening the fruit when dry and puffed, the porous structure was preserved thereby producing a tender puffed product. This invention, patented in U.S. Pat. No. 2,110,184, issued Mar. 8, 1938 still required two drying steps to be used. According to curve 30, the fruit was prepared with sulfur compounds and dried to about 20% moisture in a conventional kiln, and then was dried in a second step, curves 32-36 in vacuum, to produce a highly successful staple of commerce called "Apple Nuggets". This two-step process, requiring labor for manipulation of trays and high energy consumption, at first not an important item in production costs, is now a disadvantage, because labor and energy have become major items of cost of production with present-day fuel prices and labor making the price of such products beyond the means of the mass market.
In the effort to reduce manufacturing cost and at the same time to produce a superior low-moisture puffed fruit, without the use of SO.sub.2 and to carry out the dehydration in a single drying step, Webb developed further improvements in process and apparatus. In U.S. Pat. No. 2,473,184, issued June 14, 1949, he shows an improved process that is carried out in his apparatus of U.S. Pat. No. 2,587,939, issued Mar. 4, 1952, and supplies the product described in U.S. Pat. No. 2,283,302, issued May 19, 1942. By the foregoing process, fresh slices of fruit containing about 80% moisture can be dehydrated to about 21/2% moisture according to the schedule of curve 38, in about two hours. The fruit was discharged with a substantial film of oil clinging thereto, which was objectionable. Also, the process required hand labor for filling and emptying trays of fruit and for the skilled manipulation of the many valves regulating the process events. Heat for the process came as steam from a high pressure boiler which burned furnace oil or the like.
In the performance of vacuum dehydration of foods using a liquid heat transfer medium under the present state of the art, no attempt is made to utilize inexpensive heat sources such as solar rays, geothermal or by-product steam, which sources are available to supply large quantities of heat at relatively low cost and low temperatures, that is, at 125.degree.-65.degree. C., or even lower temperatures. Also, no improvement has been added to the art of inexpensively condensing the vapors that arise from foods being dehydrated in such vacuum dehydrators.
Thus Lankford, in U.S. Pat. No. 3,718,485, issued Feb. 27, 1973, condenses his vapor on refrigerated coils. Such condensation requires operation of a prime mover to provide energy to compress the refrigerant, and the energy which is put in the oil heater to heat the liquid heat transfer medium must eventually be removed from the system by the refrigerator. Owing to the multiplicity of heat transfer means employed, from the steam heater to the oil, to the food particles, thence to the vapor arising out of the food, thence to the refrigerating coils and the refrigerant cooling coils of the compressor, the prime mover that drives the compressor must provide several times more energy into the refrigerator compressor than is supplied by the steam oil heater. Refrigerant energy is one of the most costly forms of energy, and Lankford uses it most inefficiently to transfer heat.
Many inventions in the "freeze-dry" art have been made of recent years whereby excellent fruit and vegetable products are preserved through sublimation of moisture from the frozen comestible under vacuum conditions of 1 torr and even lower absolute pressures. To reach these extremely low pressures commercially requires massive vacuum pumps and high-capacity refrigeration systems to cool surface condensers to cryogenic temperatures. Such systems are highly consumptive of energy and capital. Energy cost, interest on capital expenditures, and maintenance, plus hand labor necessary for loading and unloading food trays, make freeze-dried foods so costly to manufacture that these foods can be afforded only in exceptional instances, such as to feed mountain-climbing expeditions and astronauts.
Systematic production of desired large-diameter, thin puffed wafers of ripe banana, pineapple and the like, has been thwarted by slices sticking or abrading during processing.
Batch methods for carrying out dehydration processes are described in the aforementioned U.S. patents. Among the disadvantages that are inherent in batch methods is the requirement that the single vacuum system absorb water vapor over the whole range of dehydration, from normal atmospheric pressure of 760 torr to the final pressure of 3 torr. A single system cannot be as efficient as a multiple system of the present invention where each vacuum component operates continuously over the range within which it is the most efficient.